The present invention relates to battery charging systems, and, more particularly, to battery charging systems in which the charging current and voltages are controlled to efficiently charge batteries without subjecting the charging circuitry to over-current operation or overcharging the batteries.
The trend in the use of rechargeable batteries in various types of tools and appliances has been increasing in the direction of heavier current, higher power applications. In the past, nickel-cadmium systems have enjoyed predominant use in hand-held power tools and other appliances. Nickel-cadmium systems, in general, have a long life in terms of the number of usable charge/discharge cycles but have relatively low energy per unit volume and a comparatively high initial cost. Where higher power devices are used, for example, in the more powerful motor driven hand-held power tools, traditional nickel-cadmium systems with sufficient power may not represent the optimum design choice from the cost standpoint. More cost efficient systems are available in other types of chemical battery systems including lead-acid systems which can provide comparatively high currents in compact and cost effective packages. The cell voltage of a lead-acid cell is about two volts as compared to 1.2 volts for the traditional nickel-cadmium cell and, accordingly, a lower number of lead-acid cells is required for a specific application relative to nickel-cadmium cells. In this regard, compact, gel-electrolyte or absorbed electrolyte lead-acid systems are available to provide sufficient power for driving the electric motors used in the larger hand-held power tools and other battery powered appliances.
The requirements for a battery charger designed for charging lead-acid cells are somewhat different from that typically considered as appropriate for nickel-cadmium systems. The impedance of lead-acid batteries varies greatly between a low, fully discharged impedance value and a relatively higher, full-charge impedance value. In addition, lead-acid batteries can be subject to sulfation in which a small fraction of the lead sulfate created during discharge is not driven into the electrolyte during charging, the sulfate tending to accumulate with time and the total number of discharge/charge cycles. Traditional battery chargers for lead-acid batteries include a step-down transformer that steps the input power (e.g., 120 VAC) down to a lower voltage that is rectified and presented to the battery through a SCR circuit. Where a high charge current is desired, e.g., on the order of four or more amperes, an adequately sized step-down transformer represents a relatively heavy and expensive component. However, a large transformer would not be considered as desirable for the physically small and lightweight chargers typically associated with hand-held tools and appliances. Even where the charger circuit is capable of providing a relatively high charge current, it is important that the current provided through the battery at the end of charge not be developed by presenting a high charging voltage, since this can unduly diminish the total number of discharge/charge cycles of the battery. Thus, in the design of a battery charger for higher current systems, it is important that the charge current be limited to a level that does not overload the charging circuit components and that the voltage levels presented to the battery be limited to a level that does not diminish the number of discharge/charge cycles or compromise the operational efficiency of the battery.